Previous studies have demonstrated that the normal distribution of pulmonary blood flow is inverted following induction of hemodynamic pulmonary edema. It is believed that the cause of this altered blood flow distribution is an increased vascular resistance in the dependent regions of the lungs. However, the exact site of this increased vascular resistance is still not known. While light microscopy of rapidly frozen lungs has been conspicuously successful in relating pulmonary structure and function, the light microscope does not provide the resolution needed to fully analyze the detailed morphological events accompanying hemodynamic pulmonary edema. Rather, the higher resolution of the electron microscope is needed to examine details of pulmonary capillary morphology. Previous studies by electron microscopy have relied on instillation of fixative into the airways or blood vessels which may alter capillary morphology. Recently I have developed techniques for preparing rapidly frozen lung for electron microscopy. Using these techniques, I propose to examine the detailed morphological events occurring during hemodynamic pulmonary edema. In addition, the effects of lung volume and pulmonary vascular pressures on the morphology of pulmonary capillaries in edematous lungs will be analyzed. The morphology of both the extra-alveolar and alveolar vessels will be examined. I believe that electron microscopic analysis of the morphology of pulmonary capillaries in rapidly frozen edematous lungs will provide a better understanding of the events responsible for the inverted blood flow distribution seen during hemodynamic pulmonary edema. Furthermore, electron microscopy of edematous lungs rapidly frozen at different lung volumes and vascular pressures will help to clarify the interaction of the various factors affecting pulmonary capillary morphology and therefore pulmonary gas exchange.